The chucks commonly used in high speed dental handpieces for releasably holding dental burs in place have chucking actions that can be generally categorized as falling within one of two groups. The first group includes wrench-operated chucks in which wrenches are utilized either to exert pushing or pulling forces to tighten such chucks. The second group includes spring grip chucks in which the bur-retaining force is generated by the spring action of the chuck and in which a wrench or some other means is utilized to open the jaws when bur removal or insertion is desired.
West German Patent DE 34 02 635 discloses a dental handpiece having a chuck with spring jaws that exert a gripping force on a dental bur. The jaws may be shifted into bur-releasing positions by depression of a push button at the upper end of the handpiece head. When the button is depressed, a wedge element is urged between the jaws to spread them apart and release the bur. However, field experiences with such a construction reveal operational problems that, until the present invention, have defied effective solution.
Bur slippage during handpiece operation is one such problem. Should such slippage occur, that is, should there be differences in the rotational speeds of a chuck and bur, the gripping jaws of such a chuck tend to wear rapidly, particularly in view of the high rotational speeds attained by modern high-speed handpieces (300,000 rpm or more). If such wear of the jaws, the bur, or both, is severe--and such a stage of wear may develop quickly--then there may be a significant risk of the bur releasing from the chuck while the turbine rotor continues to rotate at high speed. Recent efforts to reduce such slippage problems and their attendant risks have included providing the jaws of such a chuck with hardened metal tips; however, tempering or otherwise hardening the tips of such jaws is believed to be relatively expensive and of only limited effectiveness in reducing the slippage problem.
Another problem characterizing the type of chucking mechanism disclosed in the aforementioned German patent is the difficulty of opening the jaws of the chuck when bur release or insertion is desired. The spring jaws should grip a bur with sufficient force to avoid slippage, but efforts to generate such gripping forces have resulted in handpiece constructions which make it difficult if not impossible for an operator to apply sufficient force to the push button to cause bur release. In actual practice, dentists utilizing a handpiece of the type depicted in the patent commonly invert such a handpiece to press its release button forceably against a table surface or some other relatively immovable surface when bur removal or replacement is required.
In the past, the gripping force exerted by a chuck, whether of the wrench type or spring type, has often been determined by measuring the pull-out force needed to axially extract a bur from the jaws of a handpiece while the turbine is at rest. If the static pull-out forces are relatively high, it has been assumed that under dynamic conditions the releasing or slipping forces will also be relatively high. One aspect of this invention now lies in the discovery that such a correlation may not exist.
Other patents illustrating the state of the art are U.S. Pat. Nos. 4,536,157, 4,575,338, and 4,089,115.